The
Rhipsalis Riddle - or the day the cacti came down from the trees
Part 1Copyright, Dr.
Phil Maxwell, Bathgate's Road, Waimate, South Canterbury (Copyright November
1998. Reprinted with permission of the author. This article originally appeared
in the New Zealand Cactus and Succulent Journal.) email: philsue@voyager.co.nz

"Rhipsalis is a highly specialized genus, unlikely to have developed
during the Mesozoic and certainly neither ancestral to the rest of the
family nor even related closely to the ancestral stock." Benson 1982:
115

"The Rhipsalidinae certainly yield in antiquity to no other cactus.
That they are 'derivative' is plainly impossible." Croizat 1961:
759

INTRODUCTION
Several years ago I was browsing in a bookstore when I came across a book
on dinosaurs I hadn't seen before. Most dinosaur books make me yawn, but
this one had quite attractive though stylised paintings, and one in particular
made me pause as it showed dinosaurs disporting themselves near an Opuntia.
This made me sigh as everybody "knows" that cacti weren't around
in the Mesozoic. I closed the book hurriedly and left the store, but I
couldn't help thinking about that illustration. Could the artist be right
after all? Cacti have no fossil record so how would we know if any were
contemporaneous with dinosaurs? In fact we can infer a great deal about
the phylogeny (ie the genealogy) of organisms without any fossil evidence
at all, as I hope to show in this article.

Most books or articles on cacti mention the fact that members of the
family are almost entirely restricted to continental North, Central and
South America and the islands of the Caribbean. There are only two exceptions
- a couple of endemic genera (Brachycereus and Jasminocereus) and several
endemic species (or subspecies) of Opuntia on the Galapagos Islands (about
800 km west of Ecuador), and the genus Rhipsalis, which as well as being
present in the rain forests of much of South and Central America, occurs
throughout a large part of central Africa, Madagascar, the Comores, Seychelles,
Mascarenes and Sri Lanka. What is remarkable is that most publications
devote little or no space to a discussion of the occurrence of Rhipsalis
in the "Old World", or if they do write it off as dispersal
by birds. Rowley's discussion (1978) is rather more critical than most,
but it is far too brief (and inconclusive). Benson (1982: 114-116) devoted
more space to the problem than other authors, and although I don't accept
his conclusions he did at least discuss the competing ideas in some detail.
However, I think it is time to look afresh at the problem in the light
of modern ideas on historical biogeography.

SOME DEFINITIONS
Biogeography is, to put it as simply as possible, the study of the distribution
of organisms (including extinct ones). [Some authors use terms such as
"phytogeography" and "zoogeography" for the study
of the distributions of plants and animals respectively, but what does
one do about fungi or bacteria or other groups that are neither plants
nor animals?] Biogeography is usually divided into two disciplines that
are certainly not completely separate, but tend to be studied by biologists
with different agendas. One is "ecological biogeography" which
deals with the ecological factors influencing the numbers and types of
organisms living in a given area - it has an offshoot devoted to the "evolving
relationship between humans and their environment" (Allanby 1985).
This has a good deal to do with current concerns about "biodiversity"
and deciding how many species can be contained within areas set aside
for wildlife and the like.

I certainly don't want to play down the importance of ecological biogeography,
but my own interest is in the other branch, "historical biogeography",
which seeks to explain how organisms come to have the distributions they
do (and by extension, the reasons why extinct groups had particular distributions).
It is a study of fundamental importance in evolutionary theory, even if
it has been downplayed by some contemporary biologists.

A LITTLE HISTORY
The scientific exploration of the planet by Europeans really got under
way during the 18th century, even if the various expeditions were not
exactly free from commercial, political and military agendas. Prior to
this few biologists ever thought very seriously about biogeography: plants
and animals occurred where God created them, or spread after disembarking
from Noah's Ark. However, it soon became clear that organisms do not occur
randomly across the globe. Why, for instance, are there so many marsupials
in Australia and New Guinea, and in South America, but except for the
Virginian Opossum in North America, none in the rest of the world? Plants
often have strange distributions too, an obvious example being Nothofagus
(southern beech), which occurs in New Zealand, Tasmania, New Caledonia
and South America. The study of fossils (paleontology) also came to be
taken seriously in the late 18th century and only added to the confusion,
for it soon became apparent that some groups of plants and animals had
distributions in the distant past very different from today.

What proved to be the most important of all the intellectual voyages
of discovery didn't even have the pretence of being for scientific purposes.
This was of course the voyage of HMS Beagle from 1831-6, which set out
to map the coast of South America as accurately as possible for the benefit
of the Royal Navy. Charles Darwin was invited to take part as a gentleman
companion for Capt. Robert Fitzroy (later to be Governor of New Zealand
from 1843-45). He set off with two major pieces of intellectual baggage
- the fixity of species, and a stabilist view of geology. One of the books
he took with him was the first volume of Charles Lyell's "Principles
of Geology", one of the most influential scientific works of the
19th century.

In Patagonia he unearthed the skeleton of a giant sloth and wondered
how it was that such a large animal had become extinct so recently. He
also collected cacti, including one later described as Opuntia darwini,
but in general he was less interested in plants than in animals and geology.
His experiences in the Galapagos - where the governor of the islands told
him that he could tell just by looking at a giant tortoise which island
it came from - eventually led him to abandon the widely held idea that
species were immutable and to formulate his theory of evolution by natural
selection.

Darwin, however, never seriously questioned geological stabilism. Geologists
in the 19th century certainly didn't deny the evidence for vertical movements
of the Earth's crust (ie in earthquakes and by extension, mountain building),
but maintained that the continents had remained in the same relative position
throughout time. This was to pose all sorts of problems for biogeographers
throughout the 19th century and beyond. There were only three options
open to those with a stabilist view.

First, there was the idea that the present ocean basins were formerly
occupied by land-masses which for reasons unknown had later foundered.
Oceanographers have searched in vain for evidence for sunken continents,
although this has never daunted the more extreme proponents of lost continents
such as Atlantis and Mu.

Second, the continents were connected by land-bridges which allowed plants
and animals to disperse (or "migrate") before becoming submerged.
It wasn't a preposterous idea of course - there is a perfectly good land
bridge between North and South America, and the remnants of one between
north-west North America and north-east Asia. However, there simply is
no evidence for most of the other land-bridges proposed by biologists.

The third approach ignores geological explanations entirely and explains
what are known as disjunct distributions by passive dispersal of organisms
by such means as wind or ocean currents, or by other organisms such as
birds. This is usually known as "jump dispersal" or "long-range
dispersal". Not an unreasonable approach of course - there are many
seeds with adaptations that allow them to be blown considerable distances,
and there are small freshwater bivalves that attach themselves to the
legs of waterfowl. Large land-dwelling vertebrates present more of a challenge,
but this has not stopped the more extreme dispersalists from coming up
with a solution - the favourite is to envisage them being transported
on rafts of vegetation carried by convenient ocean currents. This according
to some is how giant tortoises and the ancestors of the land and marine
iguanas got to the Galapagos. A few years ago a locally produced TV documentary
used a similar explanation to account for the presence of an iguana in
Fiji - its ancestor was alleged to have drifted on such a raft for several
thousand kilometres across the Pacific from South America! Darwin himself
spent much time immersing seeds in salt water to see how long they could
remain in the oceans until they found suitable landfall. Some seeds have
an impervious coating but others soon became water-logged. [Nothofagus
seeds fall into the latter group.] Dispersalists admit quite freely that
some of the processes they envisage are highly improbable but argue that
given enough time almost anything is possible - not a very satisfactory
explanation. After all the molecules making up my computer keyboard could
in theory all move upwards at the same time and take it through the ceiling,
but I have no intention of attaching it to the bench with superglue! The
dispersalist theory is often called "centre of origin" biogeography:
a species evolves in some restricted area, then disperses far and wide.

Of course, there is a very viable alternative to stabilist geology. The
story should be familiar by now, how the German meteorologist Alfred Wegener
proposed the idea of Continental Drift in 1912 partly to explain the similarity
in shape and geology between the Atlantic coasts of South America and
Africa; how it was vilified by most earth scientists for about five decades
until the mid-60s when some inspired geophysicists came up with the idea
of sea-floor spreading and later, plate tectonics, all to the immense
discomfort of traditional geologists. Of course things were never quite
that simple - in fact it was a group of geologists in the southern hemisphere
who kept Wegener's theory alive; Alexander du Toit in South Africa, Warren
Carey and the New Zealand-born Lester King in Australia, and John Bradley
in New Zealand. In Britain it was Arthur Holmes, who even came up with
a mechanism for continental drift. Geophysicists in the meantime were
more noted for "proving" continental drift was impossible. The
theory held on in the southern hemisphere because the evidence has always
been strongest here - the concept of Gondwana and its break-up have remained
the cornerstone of the theory. It is also worth noting that some biologists
were aware of the implications of continental drift long before plate
tectonics became accepted. One who is relevant to our story is the American
botanist W.H. Camp who published a paper in the Journal of the New York
Botanical Gardens in 1948 titled "Rhipsalis - and plant distributions
in the Southern Hemisphere", in which he explicitly attributed the
distribution of this genus to continental drift. Another early proponent
of the idea that Rhipsalis is a "Gondwanic" genus was Croizat
(1952:362, cited by Hunt 1967:433).

A BRIEF DIGRESSION
I'm now going to digress ever so slightly and say something about one
of the most controversial biologists of the 20th century, the aforementioned
Leon Croizat. I'm doing this because he has been ignored by the biological
establishment for far too long and is overdue for reassessment. The following
information on his life is taken from Hull (1988).

Croizat was born in 1894 in Turin, Italy of French parentage; his parents
separated when he was 6, and he spent the first half of his life in poverty.
He emigrated to the United States in 1923, made a living selling his watercolours
until the Crash of 1929, moved to Paris where he found the life as a penniless
artist less than satisfying, and then moved back to New York. He was eventually
employed as technical assistant to the Director of the Arnold Arboretum
at Harvard. He is of particular interest to cactophiles because many of
his early publications dealt with succulents and were published in the
American Cactus and Succulent Journal. One publication, however, is a
141 page booklet "De Euphorbio antiquorum atque officinarum"
(A study of succulent Euphorbiae long known in cultivation), dated 1934,
which seems to have been privately published, a harbinger of what was
to come. [For the record he proposed the cactus genus Navajoa in 1943.]
He had the temerity to publish a paper critical of a leading Kew botanist
and was in time sacked, which I suspect left him with an outsized chip
on his shoulder. He then emigrated to Venezuela, had several jobs in botany
in academia between 1947 and 1952, divorced his first wife and married
a Hungarian refugee who ended up owning the most successful landscaping
firm in Caracas. This was important because his major works, including
"Panbiogeography" (3 large volumes), "Principia botanica"
(2 volumes) and "Space, time form: the biological synthesis"
were privately published. These books, totalling several thousand pages,
are not for the faint-hearted; in fact, it is fair to say that Croizat's
vitriolic attacks on his opponents have put many readers off. Attitudes
to Croizat have not been helped by some of his disciples - many of whom
happen to be New Zealanders - who have adopted his tactics. Croizat, who
died in 1982, coined the slogan "earth and life evolve together".

Croizat's great insight came when he plotted distributions of closely
related organisms (eg members of a genus) on a map of the world. He soon
found that similar patterns ("tracks") emerged for quite disparate
groups, whether they were trees, lizards or insects. The conclusion he
came to, and the one which is the least controversial of his claims, is
that these patterns are not the result of chance dispersal but reflect
a far more basic underlying cause. Widely distributed organisms were likely
to be of ancient origin. He didn't rule out chance dispersal, but relegated
it to a minor role to account for the rare exceptions. Most plants and
animals in fact have limited powers of dispersal. In 1974 Croizat published
a joint paper with two biologists with the American Museum of Natural
History, Gareth Nelson and Don Rosen, called "Centers of origin and
related concepts". It was a critique of dispersalist scenarios and
introduced the world to what is usually called "vicariance biogeography."
This claims that after a species arises it spreads quite rapidly into
the available space, bounded only by ecological requirements. Any disjunct
distribution arises from "vicariance", splitting the original
range by climatic, geographical or geological processes. At the smallest
scale this may be the result of a river changing its course, by mountain
building, or by drowning of coastal hills to form isolated islands (eg
in the Marlborough Sounds). On the largest scale rifting of the range
is caused by continental drift. The paper appeared at a good time - plate
tectonics was accepted by nearly all earth scientists and biologists by
the early 70s, but nonetheless it proved very controversial. One implication
of the theory is that - other things being equal - the most widely distributed
member of a group of organisms with limited powers of dispersal will be
the oldest member of that group.

Croizat later repudiated his paper with Nelson and Rosen, claiming that
his particular kind of biogeography, which he called "panbiogeography",
was not the same as vicariance biogeography. In fact, vicariancism tends
to focus on the continental masses, whereas panbiogeography regards the
ocean basins as all-important. Croizat had pointed out that the margins
of continents were at least in some cases composite, that they seemed
to have been added to over long periods and retained their distinctive
biotas. One example was the Pacific coast of South America. During the
1980s geologists came up with the concept of "terranes". Work
in Alaska revealed a large chunk of the country (called Wrangelia) that
seemed to have a very different geological history from that of adjacent
rocks. It was suggested that Wrangelia had formed a long way (thousands
of kilometres) from its present position and had been "accreted"
much later. It was soon found that similar "exotic" or "suspect"
terranes are widespread, particularly around the Pacific. The terrane
concept has since become part of geological orthodoxy, and the adjectives
have long since been dropped. New Zealand itself is now thought to be
made up of several terranes added onto a small continental core.

THE RHIPSALIS PROBLEM
Now that I have given the background, let's take a look at Rhipsalis.
The Rhipsalis occurring outside the Americas are closely related to the
very widely distributed R. baccifera (J.S. Mueller) Stearn [long known
by its synonym R. cassutha (or cassytha) Gaertner]. In fact, Barthlott
& Taylor (1995: 63-65) regard R. baccifera as a polytypic species
which they subdivide into 6 subspecies, including:

R. baccifera subsp. horrida (Baker) Barthlott which is known only
from Madagascar.

[The other subspecies are restricted to South America.] They do not give
reasons for this classification, and I prefer to regard these "subspecies"
as distinct species if only to avoid clumsy trinomials. In fact, R. mauritiana
is tetraploid (ie has twice the normal number of chromosomes, which is
22 in this group) whereas R. baccifera includes both diploid and tetraploid
forms; according to Barthlott & Taylor (1995:64) the former also differs
in "micromorphological epidermal characters and in having generally
larger fruits". R. horrida may be tetraploid or octoploid (with four
times the complement of chromosomes) and differs in having ribbed adult
stems with bristle-like spines.

There are three suggests that have been made to account for the distribution
of Rhipsalis. These are (a) introduction by humans, (b) long-range dispersal
by natural processes, and (c) vicariance.

(a) Introduction by Humans
This is assumed to have taken place in post-Columbian times (ie within
the last 500 years), as there is no evidence that Amazonian Indians ever
crossed the Atlantic (or that Africans made return trips to Brazil). Benson
(1982: 115) took this possibility seriously enough to discuss it in some
detail, and commented "Rhipsalis is both a beautiful plant and a
curiosity, as a succulent, leafless epiphyte living on and dangling gracefully
from tree branches, logs, or cliffs. It was among the first plants to
capture the attention of explorers of the tropics." [Was it? - Benson
does not back up this claim.] He went on to suggest that R. baccifera
is: "the most widespread and abundant species in Latin America and
the one universally cultivated, its escape from cultivation in two widely
separated parts of the Eastern Hemisphere tropics, and its rapid dispersal
there seems likely. R. baccifera could have invaded disturbed areas or
even native forests with ease. The small, fleshy mucilaginous fruits of
Rhipsalis are eaten or carried on the bills, shanks, feet, and feathers
of nonmigratory as well as migratory birds, and the escape of this genus
from cultivation to the surrounding forests is likely. There have been
nearly five centuries during which this could have occurred."

Of course there are plenty of examples of introduced plants and animals
invading native habitats, and New Zealanders certainly don't have to look
far for examples. However, Benson's argument makes a lot of assumptions
that are not documented. One is that R. baccifera is "universally
cultivated". Is there any evidence that Rhipsalis of American origin
was ever cultivated in west Africa prior to the 20th century? What about
Madagascar? And what about the Comores, Mascarenes, Seychelles and Sri
Lanka, most of which are a long way from standard shipping routes? R.
baccifera is in fact a rather inconspicuous plant with tiny flowers and
white fruits, and surely would not have been the species of choice for
cultivators. [It certainly doesn't seem to be at present either - most
popular books on cacti barely mention Rhipsalis - if at all - and if they
do usually illustrate other species.] More likely candidates for cultivation
would be species with more striking flowers such as R. grandiflora or
R. megalantha.

And is a period of five centuries really long enough for an epiphyte to
spread across tropical Africa (a distance of about 3500 km)? In fact,
the time available is a lot less as the occurrence of Rhipsalis in the
eastern hemisphere has been known for more than 170 years. The real problem,
however, is that the Old World Rhipsalis are not identical with typical
R. baccifera, and that there are in fact good grounds for regarding them
as distinct species (see above).

There is a variant of the human-vector scenario that I will dispose of
quickly and mention only for its risibility. I don't know who first suggested
it but it gets mentioned every now and then if only for laughs (eg Rowley
1978: 5). It is suggested that homesick sailors used Rhipsalis as a substitute
for mistletoe on long sea voyages; after all as Rowley points out, there
is a superficial similarity between R. baccifera and common mistletoe.
[Rowley does not specify the nationality of these mythical seamen, but
Cullmann, Götz & Gröner (1986: 49) claim they were English!].
The mind boggles at the thought of marines attaching Rhipsalis in cabin
doorways, but you know what sailors are! The main problems are the wide
distribution of Rhipsalis in places like central Africa, far from any
port, and on tiny islands such as the Comores, and of course the fact
that several different taxa (probably distinct species) are involved.

(b) Dispersal by Birds or by Rafting
The most popular explanation, if the books available to me are any guide,
is that birds dispersed the seeds of Rhipsalis across the Atlantic then
to Madagascar, Comores, Seychelles, Mascarenes and Sri Lanka. This certainly
has a superficial plausibility - Australian birds often get blown across
the Tasman Sea to New Zealand (a distance of about 1700 km) but I don't
know of any evidence that they bring seeds with them. One who explicitly
attributed the disjunct distribution of Rhipsalis to long-range transport
by birds was R. Roland-Gosselin whose original paper appeared in 1912
and was reprinted in 1947. Anthony (1948, cited by Benson 1982: 115) supported
this theory in a paper published in the same issue of the Journal of the
New York Botanical Gardens as Camp's. Similar ideas have been espoused
- invariably with little or no critical discussion - by many subsequent
authors, although Benson (1982: 115) was commendably cautious when he
commented "transportation by birds is a somewhat plausible explanation,
though there is no evidence that it is the correct one". The leading
worker on the genus, Wilhelm Barthlott (1979: 23) stated "migrating
birds must presumably have brought seeds to West Africa, some thousands
(or millions) or years ago, and these plants spread from there".
In their generally excellent book "The Cactus Primer" Gibson
& Noble (1986: 250) mentioned the fact that some biogeographers had
pointed to the distribution of Rhipsalis as evidence for continental drift,
then stated:

"This example was thoroughly discredited by biologists, who noted
that the species involved, R. baccifera, is widespread in the New World
and the Old and because birds feed on the small, sticky, whitish fruits
and then deposit the seeds with their feces a long distance from where
the fruits were eaten."

There are two points to make about this claim - the first of course is
that the New World Rhipsalis differ from the typical R. baccifera (see
above). The other point is that Gibson and Nobel do not provide any documentation
for the efficacy of bird dispersal of Rhipsalis seeds. Exactly how far
is "a long distance" - 10 km, 50 km or 1000 km? I for one would
like to know if any experiments have in fact been carried out to answer
this question. Rowley (1978: 4-5) was one of the very few botanists to
have devoted more than a few words to the Rhipsalis problem - he mentioned
the three possibilities and commented that "Roland-Gosselin rather
too blandly accepted birds as the agency responsible" for its presence
in the Old World, but he did not come down firmly in favour of any theory.

The first problem with the dispersalist scenario is to identify a suitable
vector. There are of course plenty of migratory birds, but most of these
fly more or less along the meridian (ie north-south and back). After all
they migrate to seek food and a suitable breeding site, leaving the northern
hemisphere in the winter to take advantage of the southern summer, and
vice versa. This is at right angles to the kind of migration we are seeking.
Some oceanic birds such as albatrosses of course fly in a more or less
latitudinal direction, but they are not noted for eating berries of any
description, let alone those on epiphytes growing in rain forests. For
the sake of argument, however, let us imagine a small fructivirous bird
eating Rhipsalis berries somewhere in Brazil, conveniently near the coast,
and then being caught up in a violent westerly storm. This bird is flown
across the Atlantic (a minimum distance of nearly 3000 km) without voiding
the ingested seeds, alights in a tree in a rain forest in west Africa
and promptly regurgitates or defecates them into a convenient crevice.
After a time they germinate, other birds eat the fruits and disperse the
seeds so Rhipsalis eventually becomes established in the west African
rain forests after which it spreads across the continent to the east coast.
The whole process is then repeated, and Rhipsalis colonizes Madagascar,
the Comores, Mascarenes, Seychelles and Sri Lanka. Even if we overlook
the fact that the Old World Rhipsalis differ from those in the Americas,
it doesn't require much thought to realise just how unlikely this scenario
becomes; if the only occurrence of Rhipsalis outside of the Americas was
in west Africa then it might be plausible, but to account for its presence
elsewhere requires repetition of an already highly improbable event! [As
an alternative, it could be argued that the seeds were stuck to the birds'
feet or feathers, even though birds take great pains to preen themselves
to maximise their aerodynamic efficiency. The problem is otherwise exactly
the same - the process has to be repeated several times.] Of course the
Atlantic was much narrower in the past - Africa and South America started
to part about 130 Ma (million years ago), with the equatorial region parting
company a few tens of millions of years later, but if bird-assisted dispersal
took place when they were much closer, exactly when was this - when they
were 1000 km apart, 500 km, or 50 km? Why not while they were still in
contact?

There is another aspect to this scenario that hasn't been addressed to
my knowledge. A bird flying or blown across the Atlantic would have the
advantage of a large "target", ie it couldn't avoid making landfall
somewhere on the west coast of Africa (assuming it could last the distance).
Madagascar is of course a lot smaller than Africa but is as little as
425 km from the African coast, and bird-assisted dispersal across the
intervening Mozambique Channel is not too implausible. [It is worth noting
that until humans arrived there (probably less than 2000 yeas ago) Madagascar
was home to a pigmy hippo. Its presence there suggests either that its
ancestor swam from Africa when Madagascar was a lot closer or that there
was some sort of short-lived land connection to Africa.] It is the other
occurrences of Rhipsalis that pose a real problem for the bird dispersal
theory. Consider first of all the Comores, tiny islands about 300 km east
of Mozambique. They certainly don't present much of a target for any bird
heading eastwards across the Indian Ocean. The same can be said for the
Mascarenes (Mauritius and Reunion) which are 850 km east of Madagascar,
and the Seychelles which are 1800 km east of Kenya and 1200 km north-east
of the northern tip of Madagascar. Sri Lanka is a lot larger than these
tiny islands, but it is a staggering 3600 km north-east of Madagascar.

Yet another question must be asked: Why, of all the cacti that have juicy
fruits (and are therefore potentially attractive to birds) should it be
Rhipsalis that has this wide distribution? Gibson and Nobel, quoted above
about the fondness some birds have for Rhipsalis fruits, go on to say
a mere two pages lager (1986: 252): "even though cacti generally
have juicy fruits, long-distance dispersal of most tribes of Cactoideae
has apparently not been effective in causing these cacti to range widely
between the continental areas or even within a continent."

Quite! Croizat's comment is particularly relevant to this discussion:
"it remains to be seen why 'casual dispersal' could not fill with
Rhipsalidinae the Andes throughout, where ecology to suit is certainly
not wanting" (Croizat 1961: 759).

If there is one cactus genus that might plausibly be subject to trans-oceanic
dispersal by birds it is surely Melocactus, some species of which conveniently
live in coastal areas, and of course have fruits which are not only juicy
but brightly coloured as well and are therefore presumably even more likely
to be eaten.

The alternative method of dispersing Rhipsalis across the Atlantic ocean
is by rafting on vegetation washed into the sea by storms. This has had
much less attention that bird dispersal, but Benson (1982: 115) thought
that "Rhipsalis would be more likely than most plants to cling to
a tree trunk or to its branches, above water". Well, maybe, but I
think it highly improbable.

In any case, as Benson admits, there are not suitable ocean currents that
would take Rhipsalis into the Indian Ocean to Madagascar, let alone Sri
Lanka and assorted islands.

It should be clear from the preceding discussion that I consider the bird-dispersal
scenario to be as dead in the water as any rain forest bird that tries
to fly the Atlantic. This really leaves only the vicariance explanation
as a viable option, and I will discuss this scenario and its implications
for cactus evolution in the second part of this article.